(A) Field of the Invention
The present invention relates to a method for correlating the line width roughness (LWR) of gratings and a method for measuring the same.
(B) Description of the Related Art
With the rapid progress of semiconductor fabrication technology, the critical dimension (CD) has been reduced to below 100 nanometers (sub-100 nm), and lithography processes have become more and more challenging. Particularly, the importance of the lithography process in controlling LWR has become increasingly significant. Extensive research shows that measuring problems caused by LWR deeply influence the allowable error of CD of the fabrication process as the line width has become smaller, resulting in deterioration of electrical properties of electronic devices.
As for the measuring requirement of LWR, the International Technology Roadmap for Semiconductor (ITRS) points out the development trend of the size of the electronic device in the future and the corresponding measuring requirements for LWR in the year of 2005. Due to the advantages of high repeatability and reproducibility, the angular scatterometer is considered to have the potential to become the primary tool for measuring the structural parameters of semiconductor devices in the near future.
The atomic force microscope (AFM) is a measuring tool first applied to LWR. The AFM is based on the principle of utilizing the weak force between the tip atom and the sample-surface atom as feedback to keep the tip scanning at a fixed height above the sample so as to measure the ups and downs on the surface of the sample. The lateral resolution of the AFM is approximately 2 to 10 nm, and the vertical resolution is approximately 0.1 nm. However, a disadvantage of the AFM is that the scanning speed is quite low, and thus it is not suitable for in-line metrology.
Another tool for the LWR is a scanning electron microscope (SEM), which has the advantage of being able to clearly show the surface roughness (for example, the uneven surface). The SEM uses the interaction of the secondary electrons between the electronic beam and the grating under test to measure the surface roughness, with a lateral resolution of approximately 5 nm and vertical resolution of approximately 10 nm. However, if the deep ultraviolet photoresist used in advanced semiconductor fabrication processes is exposed to the electronic beams of the SEM, it can easily cause a line width shrinkage effect, thereby influencing the CD measurement.
The conventional AFM applied to the grating LWR measurement has the disadvantage of very low scanning speed, while the SEM causes shrinkage of the photoresist pattern to result in the line width shrinkage effect thereby influencing the CD measurement.